76 



watch glass containing alcohol and allow them to remain 

 there until the color has almost entirely disappeared, or has 

 become a pale gray. Rinse in water and examine at once, 

 or allow them to dry and mount in balsam. (Sections of 

 tissues must be dehydrated and cleared before mounting.) 

 Formula for Gram's solution (I/Ugal's) : 



Iodine i gram. 



Potassium iodide 2 grams. 



Distilled water 300 c. c. 



Certain bacteria stain deeply when treated by this method 

 while others are decolorized by the alcohol. On this account 

 it is of some differential value. 



EXERCISE XLV. 



BACTERIUM (BACIIyLUS) BIPHTHERIAE (continued). 



§131. General Directions. Re examine all the cultures 

 of B. diphtheriae , and describe all changes which have ap- 

 peared in them. 



Examine microscopically, in stained cover-glass prepara- 

 tions, the bacteria from the glycerine agar, blood serum, and 

 I/oeffler's blood serum cultures. Stain with alkaline methy- 

 lene-blue, and note especially the way the bacteria stain. 

 Stain a few preparations with the other staining solutions 

 and compare with the methylene-blue stain. 



Note with special care the morphology of the bacteria and 

 make a drawing of a few of them. 



Examine sections of diphtheritic membrane showing diph- 

 theria bacteria, and streptococci (furnished). Reject all 

 cultures of diphtheria bacteria. 



Inoculate from the unknown cultures furnished such media 

 as the requirements of the next exercise seem to demand. 



